Aragonite formation in confinements: A step toward understanding polymorph control

aLaboratory of Materials and Interface Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands;bCenter for Multiscale Electron Microscopy, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands;cInstitute for Complex Molecular Systems, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

aLaboratory of Materials and Interface Chemistry, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands;bCenter for Multiscale Electron Microscopy, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands;cInstitute for Complex Molecular Systems, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands

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Calcium carbonate (CaCO3) is one of the most common minerals on Earth; it not only forms rocks like limestone or marble but is also a main component of biominerals such as pearls, the nacre of seashells, and sea-urchin skeletons (1). Despite many years of research, the polymorphism of CaCO3 is still far from being understood. CaCO3 has three anhydrous crystalline forms: calcite, aragonite, and vaterite, with a decreasing thermodynamic stability under aqueous ambient conditions (calcite > aragonite > vaterite) (2). While vaterite is rare in nature, calcite and aragonite are both frequently found in rocks or biominerals (1). A well-known example is the aragonite structure of nacre (3), where the organization of the crystals leads to extraordinary mechanical performance. However, in synthetic systems, crystallization experiments only generate a small fraction of aragonite compared with calcite at ambient conditions and in the absence of additives (4). So, how is the formation of aragonite facilitated in nature, especially in biominerals? In PNAS, Zeng et al. (5) shed light on this matter by showing that aragonite formation is dramatically promoted within confinements.

In recent decades, great efforts have been spent toward understanding the strategies exploited by organisms to regulate aragonite formation, and several key factors have been identified. Up to now, the effect of Mg2+ additive is the most well established. Mg2+ is abundant in seawater and is expected to be present during the formation of many marine biominerals (6, 7). At high Mg2+:Ca2+ ratios, aragonite forms as the major crystalline …

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